Wound golf balls are the preferred ball of more advanced players due to their spin and feel characteristics. Wound balls typically have either a solid rubber or fluid-filled center around which a wound layer is formed, which results in a wound core. The wound layer is formed of thread that is stretched and wrapped about the center. The wound core is then covered with a durable-cover material, such as a SURLYN.RTM. or similar material, or a softer "performance" cover, such as Balata or polyurethane.
Wound balls are generally softer and provide more spin than solid balls. This enables a skilled golfer to have more control over the ball's flight and final position. Particularly, with approach shots into the green, the high spin rate of soft-covered-wound balls enables the golfer to stop the ball very near its landing position. In addition, wound balls exhibit lower compression than two-piece balls. Their higher spin rate means wound balls generally display shorter distance than hard-covered-solid balls. The advantages of wound constructions over solid ones, however, relate more to targeting or accuracy than distance.
To meet the needs of golfers with various levels of skill, golf ball manufacturers also vary the compression of the ball, which is a measurement of the deformation of a golf ball under a fixed load. A ball with a higher compression feels harder than a lower-compression. Wound golf balls generally have a lower compression than solid balls, which is preferred by better players. Whether wound or solid, all golf balls become more resilient (i.e., have higher initial velocities) as compression increases. Players generally seek a golf ball that delivers maximum distance, which requires a high initial velocity upon impact; therefore, manufacturers of both wound and solid golf balls balance the requirement of higher initial velocity from higher compression with the desire for a softer feel from lower compression.
To make wound golf balls, manufacturers use automated winding machines to stretch the threads to various degrees of elongation during the winding process without subjecting the threads to unnecessary incidents of breakage. As the elongation and the winding tension increases, the compression and initial velocity of the ball increases. Thus, a more-lively wound ball is produced, which is desirable.
Some methods attempt to employ constant tension during the entire winding process by attempting to apply a constant pull or force on the thread. However, variations in thread cross-sectional area prevent balls formed under constant pull from having constant stress or constant elongation throughout the ball. For example, as the cross-sectional area of the thread decreases, the thread stretches to a greater degree given a constant pull. Conversely, as the cross-sectional area of the thread increases, the thread stretches to a lesser degree under a constant pull. This results in uncontrolled variations in stress and in compression throughout the finished ball, which may negatively affect the ball's performance.
Furthermore, to account for variations in thread cross-sectional area, manufacturers of wound balls do not wind using the maximum tension or stretch the thread to the maximum elongation, because to do so would cause an excessive amount of thread breakage during manufacture or play. This also prevents manufacturers from optimizing ball performance. In addition, the rubber elastic modulus also affects compression which is not considered when manufacturers attempt to control tension alone.
U.S. pat. No. 4,783,078 to Brown et al. discloses one method used in an effort to decrease thread breakage. In this patent, thread is wound first at low tension then at high tension. Controlling tension alone, however, is an approximate means of achieving the desired compression.
U.S. pat. No. 2,425,909 to Wilheim discloses one winding method that considers the cross-sectional area of the thread during winding. In this patent, an apparatus measures the pounds per square inch tension of a portion of thread during winding, applies the level of total tension to the thread during winding, and automatically adjusts the level according to the pounds per square inch tension measurement in order to keep the pounds per square inch tension value constant throughout the winding process.
Golf ball manufacturers are continually searching for new ways in which to provide wound golf balls that deliver improved performance for golfers while decreasing the occurrence of thread breaks both during manufacturing and during play. It would be advantageous to provide a wound golf ball with a higher compression, higher initial velocity, improved durability, and improved manufacturing processibility. The present invention provides such a wound golf ball.